A conventional magnetic storage medium includes a substrate supporting an underlayer disposed under a magnetic layer protected by an overcoating layer. These layers are usually deposited using a sputtering process. The magnetic layer is typically formed from a granular ferromagnetic material such as a cobalt alloy, in which data bits are stored. Transitions of magnetization in the tiny grains of the magnetic layer are used to store data and generate signals, which may then be read or written by a read/write head.
Magnetic layers are typically classified as being either longitudinal or perpendicular. In a longitudinal magnetic storage medium, the magnetic moments of the grains are aligned parallel to the plane of the substrate surface of the storage medium. In perpendicular magnetic media, the magnetic moments of the grains are aligned perpendicular to this plane. A longitudinal recording system uses a ring head for write operations while a perpendicular recording system uses a single pole head for write operations.
As the demand for storage space grows, there is a constant need to increase the areal density in magnetic storage media. To do so, the number of grains in a medium is increased by decreasing the size of the magnetic grains. This reduction in grain size also leads to sharper transitions and a higher signal to noise ratio (SNR), which improves data recording. Unfortunately, a major problem with reducing grain size is that it leads to decreased thermal stability.
Maintaining thermal stability of the magnetic grains is critical to data retention, particularly over longer periods of time. If the grains are unstable, then it is likely that data may be lost through magnetization decay. The energy required to store and maintain the stability of written bits of data is known as magnetic-anisotropy energy, KuV, where Ku is the anisotropy constant and V is the volume of the grain. Below a certain grain size, the magnetic layer will experience what is known as a super-paramagnetic effect due to the decrease of this magnetic-anisotropy-energy.
To ensure thermal stability of the grains and minimize the tendency for magnetization decay, a thermal stability factor is used as a reference to determine the stability of the grains with respect to the life span of the media. This thermal stability factor is defined by the magnetic-anisotropy energy over the thermal energy of the magnetic media:
                    K        u            ⁢      V                      k        B            ⁢      T        ,where T is the temperature and kB is the Boltzmann constant.
For example, a medium with thermal stability factor of 60, the industry standard, has a life span of about 3.6×109 years. If the thermal stability factor is reduced, the life span of the medium is reduced accordingly. At a thermal stability factor of 40, for example, the life span would be approximately 7.5 years, and at 25, the life span of the media is approximately 72 seconds). It is therefore important to have this factor at or above 40 to ensure that data stored in a medium resists magnetization decay over a sufficiently long period of time.
While it would be ideal to use materials that have a higher Ku to maintain the thermal stability factor while V is reduced, conventional longitudinal recording systems are limited by the ring heads that are used. Currently, the ring heads have a maximum head field of 2 Tesla. This head field limits the Ku of the digital storage medium to about 2.1×106 erg/cm3. This limit thus imposes a highly undesirable minimum threshold on grain size to maintain the thermal stability factor. In contrast, perpendicular recording systems using a single pole head are able to generate a higher field relative to that of the ring head. Unfortunately, perpendicular recording systems are subject to limitations and problems that have prevented the technology from being more widespread. In particular, excessive noise interference is caused by a soft underlayer used in a perpendicular medium. Although perpendicular media have greater thermal stability than longitudinal media, the coercivity in the perpendicular medium is considerably higher. This high coercivity requires higher switching field energy to perform write operations, which may cause further noise interference. Thus, perpendicular recording systems are not a commercially viable option.
In view of the foregoing, it is desirable to devise magnetic recording disk media that will allow increases in areal density while preventing magnetization decay. To overcome the limitations of existing magnetic recording systems, there is a need to have a magnetic recording medium that allows continued reduction of grain size while preventing excessive interference from noise. In addition, it is also desirable to have a magnetic recording medium that maintains thermal stability as grain size is reduced.